There are many applications in various fields of technology in which diffusers are employed to produce gas bubbles having a very small diameter (measured in microns) to facilitate high rate dissolution of a gas in a liquid. Among these applications are (1) froth flotation for separation of minerals and non-minerals, (2) clarification of liquids for solids removal or unwanted gas removal, (3) aerobic and non-aerobic fermentation for production of pharmaceuticals or production of chemicals, (4) waste water and water treatment for carbonaceous removal, nitrification and disinfection, and (5) other gas-liquid reactions such as hydrocarbon oxidation, hydrogenation, pH control and beverage carbonation.
In addition to these gas-liquid reactions, there are many liquid-liquid reactions that require the dispersion of very fine droplets of liquid in a body of receiving liquid. Some examples of such reactions are (1) emulsion formation, (2) nitration converting an organic compound into a nitrate, and (3) polymerization.
These many and varied applications require bubbles or droplets of uniform diameter but in various size ranges to accomplish the desired result most efficiently and with the smallest amount of undissolved or unused gas or liquid. In most applications the smallest diameter bubble or droplet is preferred, but this is not true of all applications. Froth flotation is an example of a process in which changing the bubble size and gas flow rate is necessary to improve the yield of various particle size fractions and accommodate anionic or cationic flotation processes.
It is generally a requirement for all applications that the receiving body of liquid be a substantially uniform and homogenous mixture of gas bubbles or liquid droplets, suspended solids, and receiving liquid.
Gas diffusers are known for dispersing fine gas bubbles into a body of liquid that utilize apertures in a rotating impeller blade or sparging arm that is immersed in the receiving liquid, with gas emitted through the apertures in a wall or walls defining a gas plenum within the rotating blade or arm. In these devices, the apertured surface is typically oriented more or less parallel to the plane in which the blade or arm rotates as gas is fed through said apertures from the interior plenum. The holes in the surface out of which gas is emitted to be sheared off in the form of gas bubbles are relatively large in these patents, and thus the resulting gas bubbles are much larger than the bubbles that are produced by use of the apparatus of the present invention, and are much larger than are suitable for many applications. Examples of such prior art devices are the devices disclosed in U.S. Pat. Nos. 3,108,146, 3,867,488, 3,911,064, 3,917,763 and 4,231,974.
A second group of patents discloses somewhat similar devices that utilize gas emission from capillary surfaces on rotor blades, and thus the devices of these patents produce smaller bubbles than when larger apertures are employed as in the patents mentioned just above. This second group of patents includes U.S. Pat. Nos. 3,630,498, 3,650,513, 3,992,491, and 4,228,112. Because the capillary surfaces from which bubbles are sheared in these prior art patents rotate within the liquid in which the diffusing device is immersed, these patents not only do not teach or suggest the present invention, but actually lead away from it.
Some prior art patents produce bubble shearing by impelling streams of liquid through slots having walls that define very small openings or foramina. The walls may be made of sintered metal, ceramic or other material providing capillary openings out of which the gas to be sheared into fine bubbles can be emitted. These patents include U.S. Pat. Nos. 4,024,618, 4,117,048 and 4,193,950. Devices such as those disclosed in these patents produce quite fine gas bubbles, but introduce the bubbles at narrowly fixed locations in the liquid in which the diffusing device is immersed, and therefore do not achieve as thorough a distribution of the gas bubbles as can be produced by a rotating disk diffuser, and of course do not in any event suggest a device of the latter type.
Stanton et al. U.S. Pat. No. 4,207,275 discloses an apparatus for mixing a liquid with a gas that includes a rotating disk that directs the liquid in which the disk is immersed radially outward to pass over apertures or annular slots extending around the top surface of an annular sparge ring that surrounds the rotating disk. However, this patent shows no understanding of the importance of small openings such as are provided by a foraminous surface. It also shows no understanding of the importance of avoiding turbulence in the bubble formation zone in order to minimize coalescence of the gas bubbles that are sheared off whatever apertured or capillary surface is employed, and fails to provide any of the several important structural features that comprise the fluid diffuser of the present invention.